1. Field of Invention
The present invention relates generally to semiconductor processing equipment. More particularly, the present invention relates to a scanning stage apparatus which provides for high acceleration when accuracy requirements are relatively low, and high accuracy when acceleration requirements are relatively low.
2. Description of the Related Art
Scanning stages such as reticle scanning stages are often used in semiconductor fabrication processes. Reticle scanning stages are generally used to accurately position a reticle or reticle for exposure over a semiconductor wafer. Patterns are generally resident on a reticle, which effectively serves as a mask or a negative for a wafer. In general, a reticle may be formed from a silicon nitride film on which a thin metal pattern is placed. When a reticle is positioned over a wafer as desired, a beam of light or a relatively broad beam of electrons may be collimated through a reduction lens, and provided to the reticle, which may be formed from silicon nitride film on which a thin metal pattern is placed. Portions of a light beam, for example, may be absorbed by the reticle while other portions pass through the reticle and are focused onto a wafer.
A reticle positioning stage which positions a reticle for exposure over a wafer is generally a high accuracy stage, since the positioning of the reticle is critical in ensuring that beams may be properly focused onto the wafer. FIG. 1 is a diagrammatic block diagram representation of a conventional reticle positioning stage. A reticle positioning stage 104 includes a coarse stage 108 and a fine stage 112. Coarse stage 108 is arranged to coarsely position a reticle 116, i.e., position reticle 116 near a desired position. Fine stage 112, on the other hand, is arranged to finely tune the position of reticle 116 once reticle 116 is positioned near its desired position by coarse stage 108.
Fine stage 112 is coupled to coarse stage 108, typically through actuators (not shown), such that accelerating coarse stage 108 effectively accelerates fine stage 112, while fine stage 112 may accelerate without causing coarse stage 108 to accelerate significantly. That is, when coarse stage 108 accelerates, fine stage 112 accelerates along with coarse stage 108. However, when fine stage 112 accelerates, coarse stage 108 does not accelerate as a result of acceleration of fine stage 112.
Actuators are typically positioned such that fine stage 112 may accelerate, or otherwise move, in an x-direction 120 and a y-direction 124. Coarse stage 108 is generally coupled to, or is part of, an actuator that enables coarse stage 108 to accelerate in y-direction 124. The actuators used to enable coarse stage 108 to accelerate are generally relatively high efficiency actuators that generate a relatively large amount of force, and are capable of high acceleration. Such actuators are generally relatively large, e.g., heavy, as the actuators are rated for efficiency and power.
Motion of coarse stage 108 in y-direction 124 is typically “long,” or has a relatively large amount of travel, as motion in y-direction 124 is used to scan a reticle. Motion of fine stage 112 in y-direction 124, which is independent of the motion of coarse stage 108 in y-direction 124 may have a stroke that varies in size from the stroke associated with motion of fine stage 112 in x-direction 120. In general, motion of fine stage 112 in x-direction 120 may require a substantially larger stroke size than motion of fine stage 112 in y-direction, as motion in y-direction may be complemented or augmented with motion of coarse stage 108 in y-direction 124.
As the need for semiconductors grows, the throughput requirements associated with semiconductor fabrication equipment also increases. Specifically, the throughput requirements associated with reticle scanning stages are increasing. Higher throughput requirements typically require higher acceleration in a reticle scanning stage. Increasing the acceleration capabilities of a reticle scanning stage generally results in an increase in the size of actuators used in the reticle scanning stage, as the increasing the efficiency and the power of an actuator often increases the size of the actuator. Specifically, the size of the actuator which accelerates a coarse stage, e.g., coarse stage 108 of FIG. 1, may increase, as will the size of the actuator which accelerates a fine stage, e.g., fine stage 112 of FIG. 1.
While increasing the size of an actuator used to accelerate a coarse stage may be acceptable, increasing the size of an actuator that acts between the coarse stage and a fine stage often is not acceptable. The mass associated with more efficient, more powerful actuators may adversely affect the performance of a fine stage, as the additional space needed by the more powerful actuators may not be readily available. Further, actuators which have a relatively high efficiency rating and a relatively high power rating generally have lower positioning accuracy capabilities than actuators which have a lower efficiency rating and a lower power rating.
Conventionally, increasing the throughout requirements of a reticle scanning stage may cause the accuracy associated with the reticle scanning stage to be compromised, as actuators which are capable of higher accelerations are generally more difficult to control and, hence, less accurate. In addition to being less accurate, it is also difficult to position large actuators such that a line of force associated with the actuators may cross through the center of gravity. When the line of force is not through the center of gravity of the fine stage, then a substantial “balancing mass” is required to balance the overall stage. The use of such a balancing mass may further increase the size of the overall stage. To increase the accuracy associated with a reticle scanning stage and to substantially minimize the size of the reticle scanning stage, smaller, more controllable actuators may be used with the reticle scanning stage, at the expense of acceleration capabilities.
Therefore, what is desired is reticle scanning stage which is both accurate and capable of high accelerations. That is, what is needed is a reticle scanning stage which is has a high level of accuracy and a high range of accelerations, and effectively does not require that high force, high mass actuators be used to couple a fine stage portion of the reticle scanning stage to a coarse stage portion of the reticle scanning stage.